Recently, as for a charging type for charging an image bearing member such as a photosensitive member or a dielectric member in an image forming apparatus using the electrophotographic type or the electrostatic recording type process, a contact charging type or a proximity charging type is used. In the contact charging type or proximity charging type, a charging member (charging roller) of an electroconductive roller type, for example, is provided contacted to or close to the image bearing member, and a voltage (charging bias voltage) is supplied to the charging member.
For example, an electroconductive rubber roller is contacted to the photosensitive member as the image bearing member, and is rotated by the photosensitive member, wherein a core metal as a rotation shaft of the rubber roller is supplied with a voltage so that the photosensitive member is uniformly charged. In this case, by the application of the voltage to the charging roller, the photosensitive member is charged by electric discharge produced in the fine gap between the charging roller and the photosensitive member.
It is not inevitable that the charging member such as the charging roller is contacted to the surface of the photosensitive member which is a member to be charged. The charging member and the photosensitive member may be disposed close to each other without contact but with the gap such as several 10 μm, if a dischargeable region as determined by a voltage across the gap by a corrected Paschen curve is assured. Here, the contact charging type or proximity charging type is a type in which the charging member is contacted to or disposed close to the member to be charged to produce electronic discharge in the fine gap, by which the member to be charged is charged.
As for the applying type of the voltage to the charging member in the contact charging type or the proximity charging type, there is an AC charging type in which a DC voltage and an AC voltage are superimposed. In the AC charging type, an oscillating voltage comprising a DC component corresponding to a required surface potential of the photosensitive member and an AC component having a peak-to-peak voltage not less than 2-times a discharge starting voltage is supplied to the charging member as a charging bias voltage.
That is, in the AC charging type, the charging bias voltage applied to the charging member is a superimposed voltage of the AC component and the DC component (the voltage corresponding to the target charged potential), and a waveform of the AC component may be a sinusoidal wave, a rectangular wave or triangular wave. Or, the AC component may be a rectangular wave voltage provided by periodically rendering a DC voltage source ON and OFF.
In the AC charging type, if a value of the peak-to-peak voltage, which may be simply called “AC voltage” or “charging AC voltage”, of the AC component of the oscillating voltage applied to the charging member is too small, a convergence property to the value of the DC component of the potential of the photosensitive member deteriorates, with a result of improper charging. In addition, when a discharge current value between the charging member and the photosensitive member varies as a result of variation of a resistance value due to a temperature change of the charging member such as the charging roller, and the temperature of the charging member lowers, an amount of the discharge current between the charging member and the photosensitive member is not enough, and by the convergence property of the surface potential of the photosensitive member toward the value of the DC component of the charging bias voltage, uneven charged potential of the photosensitive member is likely to occur.
As Japanese Laid-open Patent Application 2008-191620 discloses, it is known that a temperature sensor is provided in the image forming apparatus to control the charging bias voltage applied to the charging roller on the basis of the output of the temperature sensor.
However, even in the case that the charging bias voltage is controlled on the basis of the output of the temperature sensor provided in the image forming apparatus as disclosed in Japanese Laid-open Patent Application 2008-191620, the problem still exists, that is, when the charging member is a part of a process cartridge, for example, and it is exchangeable, the temperature detected by the in-apparatus temperature sensor may be different from the actual temperature of the charging member immediately after the replacement of the charging member. Particularly when the temperature of the new charging member mounted to the image forming apparatus by the replacement is lower than the temperature inside the image forming apparatus, the amount of the discharge current between the charging member and the photosensitive member is not enough, with the result of deteriorated convergence property of the surface potential of the photosensitive member toward the value of the DC component of the charging bias voltage, and therefore, uneven surface potential of the photosensitive member may occur.